Do you know if the cake bootstrapper pacakges.config you use (which looks emphatically deprecated by the way) updates apace with cake releases? If so, it seems that your builds started breaking around the new 0.28.0 release.
It's not immediately apparent to me how you could pin an earlier version of cake to test. Any ideas?
I've tried to locally rebuild several of your previous commits and various combinations and from them it seems that once the target framework changed to netcoreapp2.1 and the sdk to 2.1.300, there is some problem with cake killing .net core processes.
To reproduce choose any commit after said .net core upgrade and run the build script. Open task manager or process explorer and see .net core processes. They remain when the script returns. If you run it again, more are created. I believe this is irrespective of what version of cake is being used.
Yesterday we closed somehow similar issue with setting -maxthreads 1 for xunit console. I understand that this is different situation, because Cake and .NET Core tests are involved here, but I still have feeling that this can be issue related to multi threading. Sorry for vague comment, we will continue investigating. Any ideas are highly appreciated.